President Barack Obama has pledged to continue the fight against Islamic State militants following the first U.S.-led air strikes targeting the extremist group in Syria.

Obama spoke at the White House on September 23 before leaving for the UN General Assembly in New York, hours after the United States and five Arab allies launched their first strikes.

Pentagon spokesman John Kirby described the overnight bombings as "very successful."

The U.S. president said the United States was "proud to stand shoulder-to-shoulder" with Bahrain, Qatar, Saudi Arabia, Jordan, and the United Arab Emirates in the aerial campaign.

He hailed the support of the Arab countries, saying, "This is not America's fight alone."

He also warned that the joint fight against Islamic State (IS) will "take time," adding that Washington will "do what is necessary" to defeat the group.

In a letter to Congress, Obama said, "It is not possible to know the duration of these deployments and operations."

In New York, Secretary of State John Kerry said the United States "will not allow geography or borders to prevent us from taking action" against Islamic State, adding that more than 50 countries had joined the coalition to fight the group.

The Pentagon earlier said the United States conducted on September 22 14 strikes against targets linked to IS -- which is also known as ISIS and ISIL -- using fighter jets, bombers, remotely piloted drones, and Tomahawk missiles.

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The Syrian Observatory for Human Rights, a Britain-based monitoring group, said the air strikes targeted the northern province of Raqqa, its provincial capital, as well as the eastern province of Deir el-Zour, which border's Iraq.

Raqqa is the Islamic State militant group's main stronghold.

Rami Abdurrahman, who heads the observatory, said 70 IS militants were killed and at least 300 were wounded in the air strikes.

The Pentagon also said eight separate U.S. air strikes were aimed at a group of former Al-Qaeda operatives known as the Khorasan Group and took place in Aleppo Province.

It said the action was meant to disrupt an "imminent attack" against the United States and Western interests by "seasoned Al-Qaeda veterans" who had established a safe haven in Syria.

It said targets included training camps, an explosives and munitions production facility, a communication building, and command and control facilities.

Obama said on September 23, "We will not tolerate safe havens for terrorists who threaten our people."

Abdurrahman said 30 at least 50 fighters from the Al-Qaeda-linked Nusra Front and eight civilians, including at least two children, were killed in air strikes against a target in Aleppo Province.

Russia, Iran Protest

The State Department said the United States warned Damascus in advance "not to engage U.S. aircraft," adding that Washington did not request permission or give advance notice of the timing of the attacks.

Syrian state media quoted President Bashar al-Assad as saying he supported any international efforts to combat "terrorism" in Syria.

Russia and Iran, two allies of Syrian President Bashar al-Assad's regime, said the air strikes violated Syria's sovereignty and international law.

A Russian Foreign Ministry statement issued on September 23 said such operations require the clearly expressed consent of the Syrian government or a UN Security Council decision.

Iranian Deputy Foreign Minister Hossein Amir-Abdollahian said, "Any kind of military intervention in Syria without the demand of the government of this country and without abiding by international laws is not acceptable."

In New York, Iranian President Hassan Rohani said the U.S. policy was "nebulous and ambiguous" because it simultaneously opposed the Islamist militants, while trying to undermine the Syrian government.

It said the country's foreign minister received, via Iraq, a letter from U.S. Secretary of State John Kerry ahead of the strikes.

The strikes on Syrian territory are part of the expanded military campaign that President Barack Obama authorized nearly two weeks ago in order to "degrade and ultimately destroy" the militant organization.

Since early August, U.S. fighter aircraft, bombers, and drones have launched about 190 air strikes targeting Islamic State militants in Iraq.

The United States estimates that about two-thirds of the estimated 31,000 Islamic State militants are in Syria.

IS militants have captured large swaths of territory stretching from within Syria to land across northern and western Iraq.

They have also slaughtered thousands of people and beheaded Westerners, including two U.S. journalists and a British aid worker.